1. Field of the Invention
The invention relates to oscillators, and more particularly to a system and method for dynamically trimming a voltage controlled oscillator (VCO).
2. Description of the Related Art
A VCO is a common circuit component, particularly in a phase locked loop (PLL). A PLL circuit includes a VCO that locks to an incoming reference signal. Generally, the center frequency of the VCO, the VCO gain, and the overall tuning range of the VCO is considered in the design of the PLL.
Implementing a VCO in an integrated circuit presents trimming design considerations since the center frequency of the VCO will vary from chip to chip due to the manufacturing process. A typical method of dealing with VCO variations is hard trimming, which utilizes a one-time programmable or selectable center frequency trimming circuit in the VCO. This allows the VCO center frequency to be brought closer to a target frequency. However, with hard trimming, the VCO can only be programmed or trimmed once, and thus the VCO is susceptible to center frequency drifting due to temperature changes, aging, and other effects. To compensate for such effects, the VCO gain is increased; however, this results in a higher sensitivity to noise.
Another method of dealing with VCO variations is soft trimming. Soft trimming utilizes a VCO operable to receive a trim signal and adjust a voltage-to-frequency operating characteristic in response to the trim signal, and thus the VCO is less susceptible to center frequency drifting due to temperature changes, aging, and other effects. However, with soft trimming, the circuit implementing the VCO usually trims during a non-operational state, e.g., a calibration state, or often suffers from temporary system degradation during soft trimming, e.g., the PLL may temporarily lose lock during the trimming adjustment.
A system is provided for dynamically trimming a voltage controlled oscillator operable to receive a trim signal for adjusting a voltage-to-frequency operating characteristic of the voltage controlled oscillator and receive a tune signal to generate an output signal having an output frequency determined by the voltage-to-frequency operating characteristic. The system comprises a trim circuit operable to receive the tune signal and generate the trim signal in response to the tune signal and increment or decrement the trim signal and condition a change in the trim signal during the increment or decrement so that the voltage-to-frequency operating characteristic of the voltage controlled oscillator drifts from a first voltage-to-frequency operating characteristic to a second voltage-to-frequency operating characteristic at a rate less than a specified frequency rate.
A phase locked loop comprises a voltage controlled oscillator, a loop filter and a trim circuit. The voltage controlled oscillator is operable to receive a trim signal for adjusting a voltage-to-frequency operating characteristic of the voltage controlled oscillator, and receive a tune signal to generate an output signal having an output frequency determined by the voltage-to-frequency operating characteristic. The loop filter is operable to receive an error signal and generate the tune signal in response to the error signal. The trim circuit is operable to receive the tune signal and generate the trim signal and increment or decrement the trim signal and condition a change in the trim signal during the increment or decrement so that the voltage-to-frequency operating characteristic of the voltage controlled oscillator drifts from a first voltage-to-frequency operating characteristic to a second voltage-to-frequency operating characteristic at a rate at which the phase locked loop remains in a locked state.
A method of dynamically trimming a voltage controlled oscillator in a phase locked loop comprises the steps of determining if a voltage-to-frequency operating characteristic of the voltage controlled oscillator needs adjustment; generating a trim control signal to adjust the voltage-to-frequency operating characteristic of the voltage controlled oscillator upon a positive determination; and conditioning the generation of the trim control signal so that the rate of change of the voltage-to-frequency operating characteristic of the voltage controlled oscillator is controlled so that the phase locked loop remains in a locked state during adjustment of the voltage-to-frequency operating characteristic of the voltage controlled oscillator.